Sound recording



J. H. McLEOD 2,263,006

I SOUND RECORDING Filed March 25, 1939 "Nov. 18, 1941.

T0 LIGHT M4LVE 0R SPEAKER 13 0U TPU T TRANSFORMER FROM AMPLIFIER T0 LIGHT VALVE VIVV V llll Fig.5.

n FROM AMPLIFIER Patented Nov. 18, 1941 SOUND RECORDING John Henry McLeod, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application March 25, 1939, Serial No. 264,141

Claims. (c1. 1794003) This inventionvrelates to sound recording and reproducing systems and particularly to monitoring devices for use in such systems.

In sound reproduction and particularly in sound recording, it is common practice to bypass a portion of the signal to a set of ear phones so that the operator can judge the quality of the sound being reproduced or recorded. This operation is referred to as monitoring. It is well known that when the intensity of a sound signal becomes too great, distortion is introduced due to various factors. If during a recording, the operator doing the monitoring hears a distorted signal or one which is so loud that the recording system is liable to introduce distortion, he adjusts the volume control to reduce the signal intensity, When monitoring a signal by the usual methods, it is difiicult to detect when the intensity of the signal just reaches the distortion level and, hence, anoperator doing a large amount of monitoring, is at one time liable to be more critical than at others and thus turn out results of non-uniform quality.

It is an object of-the present invention to provide means for insuring that the signal intensity will never be permitted to exceed some predetermined value unnoticed.

It is a particular object of the inventionto provide an audible signal in the monitoring circuit which will advise the operator that the signal being recorded or being reproduced has exceeded a predetermined intensity level.

7 It is an object of one particular embodiment of the invention to provide both an audible signal and a visible signal which will announce to the operator that the distortion level has been reached.

In any sound recording or sound reproducing circuit, the signal is in the form of an alternating current or alternating potential. Distortion is introduced whenever this alternating potential exceeds some threshold value. According to the invention there is provided a monitoring circuit receiving a portion of the sound signal and provided with a glow tube which is biased to operate (1. e. to be extinguished or to light up) whenever the intensity of the sound signal exceeds a certain value. If it is desired to have the glow tube operate in the sense of being extinguished, the biasing potential is arrangedso that this operation occurs whenever the negative half of the alternating current produces negative potentials higher than a certain value. Whenever the sound signal intensity is less than this predetermined value the glow tube glows continuously andproduces no effect. When the sound signal intensity exceeds this value, the glow tube is caused to go out and to glow again each cycle of the sound. This causes a loud harsh sound in the ear phones of the monitoring circuit and the operator is immediately aware thatthe signal intensity is excessive;

According to a difierent embodiment of my invention, the glow tube is arranged in a circuit parallel to the main portion of the monitoring circuit and is biased so that it is out except when the signal intensity exceeds a certain value. In this embodiment there is provided not only the audible signal but also the visible signal due to the fact that the glow tube lightsup whenever the distortion level is reached. In some sound recording systems it is desirable to have the monitor ear phones at a considerable distance from the rest of the system and it is difficult to insure that the signal will pass with sufficient intensity through the cable leading to the ear phones. According to a special embodiment of my invention, the monitoring vcircuit-is provided with a glow tube near the source of the signal and with a second glow tube in that part of the circuit which is near-the earphones. The first glow tube insures that there will be an intense signal which will be transmitted through the cable leading to the phones whenever the distortion level is reached even though the actual signal being recorded has a relatively low intensity whether. it is normal or above the distortion level and the second glow tube insures that this distortion indicating signal will be heard in the ear phones. In this special embodiment, the original sound signal may be en tirely absorbed before reaching the ear phones, but the operator will be made definitely aware of whether or not the sound signal intensity has exceeded the distortion level. In any of these embodiments of the invention a grid glow tubemay be used instead of an ordinary two electrode glow tube. V I

In the preferred embodiment of my invention there is provided a triode or other amplifier ahead of the glow tube in the monitoring circuit so as to prevent any feed back'of the glow tube signal into the sound recording'or reproducing circuit. 1

Other objects and advantages of my invention will be apparent from the following description when read in connection with the accompanying drawing in which:

Fig. 1 shows a monitoring circuit embodying oneform of my invention. Fig. 2 shows another form ofmy invention. Fig. 3 shows an embodiment of my invention similar to Fig, 1, but employing a grid glow tube.

In the accompanying drawing, the usual symbols are employed to indicate electrical resist-. ances, capacities, etc. In Fig. 1, a sound signal carried by leads I!) is introduced by a transformer H to a recording light valve or a loud speaker circuit. According to the invention a portion of this signal is by-passed by means of a condenser 12 into a monitoring circuit including a resistance l3, an amplifier l4, and ear phones or other sound producing means. For convenience the term "ear phones is used in this specification to include loud speakers. as well as the typeheld close to the ear. The circuit includes a glow tube I6 which is biased by the plate potential supplied from a source H, to remain lighted except when the intensity of the sound signal reaches a value at which the negative half of each cycle drops to the potential at which the glow tube 16 is extinguished. .The biasing of the glow tube is adjusted so that this extinguishing potential is just that which is reached when the intensityof the sound signal is such that intolerable distortion would be introduced to the light valve or the loud speaker, or caused by them. Thus for all normal sound intensities the glow tube It remains lighted and the sound is heard in the rear phones l5. When the distortion' level is reached, there is in addition to the normal sound signal in the ear phones I 5, a very loud harsh noise due tothe intermittent glowing and extinguishing of the glow tube It. Thus there is provided an audible signal to announce to the operator that the distortion level has been reached. If necessary an auxiliary biasing potential may be placed across the glow tube 16 in the same manner as the potentiometer 25 is acrossthe tube I-B-inFig. 2.

In Fig. 2 the monitoring signal is provided for the earphones through a circuit I8. A glow tube I9 is provided in a circuit parallel to this circuit-l8 and isgconnected thereto by a twoposition switch 20. The glow tube i9 is biased by an adjustable source of potential 25 so that it remains out forall normal signals and will be lighted during a portion of each cycle only when the intensity of the signal exceeds the distortion level. Thus for all normal signals the earphones l5. receive only the signal directly through the circuit; I8,.wh ereas an excessive signal will cause the glow tube l9 to light andthus provide an additional and harsh noise in the ear phones l5. In addition to the audible signal produced in the ear phones l5 there is a visible signal provided bythe glow tube 19 itself. This visible effect is not present in the arrangement shown in Fig, 1 since it isv difficult to distinguish between the intervals wherein tube" l6 glows continuously and those when it glows intermittently at high frequency. v i

The two position switch 2? permits a selection of the signal intensity at which the glow tube 'l'9'will be caused to glow. 4 V

If theear' phones l5 are tobe at any considerable distance from the rest of the electrical cijrcurt, for example, if the leads 22 are enclosed in a long cable, the signal through the circuit I8 may .be reduced to an almost inaudible level. I-Icwever, the noise due to the operation of the glow tube 19 will be transmitted and heard through a considerably longer'cable thanthat to which the'signal itself would pass. This indicating signal may be further amplified by asecond glow tube Zl placed in that part of the circuit which isnea'r the ear phones I5. This second glow tube 2|" is biased so that if there is nosignal from the glow tube Hlthe second glow tube 2| remains out,but anyindicating signal'from the first tube I9 will cause the second tube to glow in synchronism-therewith and cause a loud noise in the ear phones [5. Similarly a second tube could be used to relay the signal from tube IS in Fig. 1, in which case the second tube would be biased to go out and relight in synchronism with the first tube. Because of the large change in output of the first tube when it operates (goes off or on) the biasing of the second tube is not at all critical.

In Fig. 3, there is shown an embodiment similar to that shown in Fig. 1, but the glow tube I6 is replaced by a gridglow tube 24, into the grid 23 of which the input signal is fed. If v the glow tube M is biased so that it goes out only when the signal intensity exceeds a certain value, the arrangement shown in Fig. 3 will operate exactly the same as in-Fig. 1 and the sound signal will be heard in the ear phones I5 when a normal signal isbeing recordedor reproduced and this soundin-the ear phones [5 will be augmented by a noisefrom the grid glow tube whenever the sound signal-intensity exceeds the'dis tortion level. If the grid glow tube- 24 is biased so that it lightsup only when the signal intensity exceeds the distortion level, no sound'will be heard inthe ear phones l5 except when the distortion level is reached. This latter arrangement provides both an audible and a visible si nal, but the parallel circuits shown in Fig. 2 have the advantage that the operator may listen to the sound signal itself as well as the indicating signal from the glow tube. Of course, grid glow tubesmay be used in place of glow-tubes l9 and 2| in Fig. 2. I

Having thus described ments of' my invention, I wish to point out it is not limited-to the specificstructures shown but is of the scope of the appended claims. 1

What I claim and desire to secure by-Iletters Patent of the United'States is:

1. In a sound recording system, a monitoring circuit including earphones, means connecting said phones to the system for operating them in accordance with a portion'of the sound carried by the recording system, and a glow tube in series with the ear phonesbiased to operate only when the signal intensity exceeds a certain value.

2. In a soundrecording system, a monitoring circuit including ear phones, means connecting the earphones to the system for operating them 1n accordance with a portion ofthe sound in the sound recording system, said circuit including two parallel paths in one of which is a glow tube biased to light only when the signal in- I tensity exceeds a certain value.

3 A monitoring circuit'according to claim .2 in which a second glow tube is placed in series with the first glow tube and in the same path of the circuit, to relay the output of the first tube to the ear phones.

4; A monitoring circuit according to claim 1 in which a; second glow tube is placed in series with the first glow tube to relay the output of the first tube to thee'arphones, said second tube being biased tooperated in synchronism with the first tube.

i 5. A monitoring circuit according to claim 1 in which a rectifier is included between the sound recording system'and the glow tube to prevent feed back of the glow tube operation eifect into the recording system. 7

JOHN HENRY MCLEOD.

the various embodi- 

